Spinal networks encode the motor outputs that generate all behaviors, therefore it is not surprising that spinal interneurons (INs) are highly complex and that our knowledge is rather incomplete about how many classes of spinal INs exist, their properties, connections and mechanisms of differentiation. This information is essential not only to understand motor function, but also how genetic alterations, disease or injury affects these circuits in adults and newborns. A few years ago a new conceptual framework to understand spinal INs was prompted by the discovery of a few canonical embryonic classes that diversify into the large variety of adult phenotypes. With previous funding we established that temporal control of neurogenesis and transcription factor (TF) expression correlates with specific IN phenotypes within a class known as V1. V1-INs include those that mediate recurrent inhibition of motoneurons (Renshaw cells) and many that mediate reciprocal inhibition of motoneurons with antagonist actions on a single joint (Ia inhibitory interneurons, IaINs). Thus, we divided V1s in an early generated group (that includes Renshaw cells and expression of MafB) and a late generated group (that includes IaINs and expression of Foxp2). However, V1 diversity is much larger and there is not yet a complete scheme of V1 IN variety and function, in part because lack of information about their output in terms of axon projections, connections and firing. Here we hypothesize that V1s of different birthdates express different combinations of TFs with specific roles in defining axonal projections and firing properties. In aim 1 we will analyze V1 axon projections and relate them to TF expression and time of birth. Aim 2 will analyze whether Foxp2 controls axon length and therefore the extent of rostro-caudal projections of different V1's. Aim 3 will analyze the firing properties of V1 groups, their relation to specific voltage-gated channels and whether these are determined by specific TFs. Validation of our hypotheses would suggest that birth-date and TF expression differentiate V1 groups with distinct connections and firing modulation. The results will also provide a more complete picture of V1s that will contribute to improve our understanding of spinal local motor circuits.